Cavity backed log periodical antenna system



D. G. BERRY 3,221,330

CAVITY BACKED LOG- PERIODICAL ANTENNA SYSTEM Nov. 30, 1965 5 Sheets-Sheet 1 Filed May 8, 1961 INVENTOR.

DAV/D 6. BER

ATTORNEYS Nov. 30, 1965 D. G. BERRY 3,22

CAVITY BACKED LOG PERIODICAL ANTENNA SYSTEM Filed May 8, 1961 5 Sheets-Sheet 2 INVEN TOR.

0A /0 6. BE Y BY 14/ A 7' TORNEYS Nov. 30, 1965 D. G. BERRY 3,221,330

CAVITY BACKED LOG PERIODICAL ANTENNA SYSTEM Filed May 3. 1961 5 Sheets-Sheet 3 IN VEN TOR.

041 0 0. BER r BY Nov. 30, 1965 D. G. BERRY 3,221,330

CAVITY BACKED LOG PERIODICAL ANTENNA SYSTEM Filed May 8, 1961 5 Sheets-Sheet 4.

IN V EN TOR.

DA V/ G. BER/P Nov. 30, 1965 D. G. BERRY 3,221,330

CAVITY BACKED LOG PERIODICAL ANTENNA SYSTEM Filed May 8, 1961 5 Sheets$heet 5 IN V EN TOR.

BYW DAT l A T TORNEKS United States Patent 3,221,330 CAVITY BACKED LOG PERIODICAL ANTENNA SYSTEM David G. Berry, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Filed May 8, 1961, Ser. No. 108,525 9 Claims. (Cl. 343719) This invention relates generally to logarithmic periodic antenna systems (hereinafter sometimes referred to as log periodic antennas) and, more specifically, to a log periodic antenna system which is comprised of a real log periodic antenna element positioned substantially flush with the surface level of the earth and backed by a cavity so that an image log periodic antenna element is created by the reflection of the real element in the cavity bottom.

Log periodic antenna elements are a fairly recent development in the antenna art. Perhaps the most important feature of the log periodic antennas lies in their ability to maintain a constant radiation pattern over large frequency changes of the order of 10 or to l, or even greater. Such antenna systems (log periodic antenna systems) may be described generally as consisting of a plurality of what is herein defined as antenna elements, each antenna element being generally triangular in shape, having a vertex, and being confined within an .angle on extending from the vertex. A boom of conductive material is positioned along the bisector of the angle a and functions to supply electrical signal to the antenna element, as well as support each antenna element. Each antenna element, in turn, is comprised of at least two radial sections, each being generally triangular in shape and having a common vertex and a common side, the common side being the aforementioned boom. The outer side of each triangularly shaped radial section is defined by a radial line extending from the vertex at an angle a/ 2 with respect to said center line or boom of the antenna element. Further, each radial section has a plurality of teeth comprised of elements which are generally transverse to the center line of the antenna element. Said teeth are all similar to one another in shape, but become progressively larger and spaced progressively farther apart as the distance from the common vertex increases. The above relationship may be expressed by stating that the radial distance from the vertex to a given point on any given tooth in a given radial section bears a constant ratio 1- to the radial distance of a corresponding point on the next adjacent tooth farther removed from the vertex than said given tooth. In the most general case, where each antenna element employs two radial sections lying in the same plane, the teeth of one of the radial sections are positioned opposite the gaps between the teeth of the other radial section. It is to be noted that throughout this specification a single tooth of an antenna radial section will sometimes be referred to as a monopole.

The log periodic antenna elements described in the preceding paragraphs may be arranged in many different combinations to perform different desired functions. Usually, the antenna elements are employed in multiples of two, that is to say, in pairs, since each antenna element must feed against another. For example, a pair of log periodic antenna elements may be arranged in such a manner that the vertices are near each other (but not quite touching), and which lie in the same plane. Such an arrangement is known in the art as a coplanar array of log periodic antenna elements. For a more detailed description of such structure, and other types of log periodic structures which provide additional background for the present invention, the readers attention 3,221,330 Patented Nov. 30, 1965 is directed to the following patent applications which are hereby incorporated by reference in the present specification: United States patent application, Serial No. 721,408, filed March 14, 1958, by Raymond H. DuHamel and Fred R. Ore, entitled Logarithmically Periodic Antenna, now Patent No. 3,079,602; United States patent application, Serial No. 804,357, filed April 6, 1959, by Raymond H. DuHamel and David G. Berry, entitled Unidirectional Frequency-Independent Coplanar Antenna, now Patent No. 2,989,749; United States patent application, Serial No. 841,391, filed September 21, 1959, by Raymond H. DuHamel et al., entitled Antenna Arrays, now Patent No. 3,059,234; United States patent application, Serial No. 841,400, filed September 21, 1959, by Raymond H. DuHamel et al., entitled Broadside Antenna Arrays, now Patent No. 2,984,835; and United States patent application, Serial No. 31,068, filed May 23, 1960, by David G. Berry, entitled Unidirectional Circularly Polarized Antenna, now Patent No. 3,113,316.

In recent years considerable thought has been given to the installation of certain military installations, and even to certain key industries, underground. The basic reason for such underground development lies largely in the fact that such installations are more difficult to detect by aerial reconnaissance either during peacetime or during wartime and, further, by the fact that it is much more diflicult to effectively bomb such underground installations should the country become involved in war. Almost all of such underground installations will re quire rather complicated communication systems. At the present time it is found quite possible to install all of the communication equipment underground, except for the antenna element. The foregoing statement should, perhaps, be modified to the extent that some antenna systems are probably available which are substantially flush with the ground level. However, such antenna systems have a rather serious handicap in that they are not broadband antenna systems.

It is a primary object of this invention to provide a broadband antenna system employing log periodic antenna elements which can be installed so that no portion thereof is above ground level.

A further object of the invention is an antenna system winch is not visible to the human eye above ground level and which is substantially immune to bombing attacks other than direct hits.

A third aim of the invention is the improvement of log periodic antenna systems, generally.

In accordance with the invention, a wedge-shaped cavity is dug into the ground which has a sloping bottom, two substantially vertical walls, and a substantially vertical end. One end of the sloping bottom of the cavity is at ground level and the other end of the sloping bottom of the cavity, ofcourse, defines the lower edge of the vertical end of the cavity. The entire cavity is sealed against leakage by some appropriate means, such as concrete or plastic liner, or other suitable means and the bottom of the cavity is then lined with a conductive sheet, such as for example, copper screening. A log periodic antenna element is then positioned parallel with the ground surface and at, or just below, ground surface and directly above the cavity with the vertex of the antenna element being positioned near the center of that end of the cavity bottom which is at ground level and the wide end of the antenna element being positioned over the deep end of the cavity bottom.

The antenna element creates an image of itself in the reflective bottom of the cavity, said reflected antenna appearing to be a distance below the cavity bottom equal to the distance that the real antenna is above the cavity bottom. In other words, the cavity bottom functions as a mirror functions to reflect light waves in the visible spectrum.

It is apparent that the angle a between the planes of the real and the reflected antenna elements, in the absence of said cavity being filled with a dielectric having a constant other than one, is approximately twice the angle existing between the cavity bottom slope and the real antenna element. Such an angular distance can be measured in wavelengths and is an important factor in determining the elevation of the radiated pattern. The larger the angular distance between the real and virtual (re fiected) images, the closer the radiated beam pattern will be to ground level.

In order to minimize the depths of the cavity for a given installation, the cavity can be filled with a suitable dielectric, such as distilled water or polyethylene, since the cavity depth in terms of wavelengths is proportional to 1/ V75, where K is the dielectric constant.

The above-mentioned and other objects and features of the invention will be more fully understood from the following detailed description of the invention when read in conjunction with the drawings in which:

FIG. 1 is a perspective view of a single log periodic antenna element positioned above a cavity dug into the surface of the earth;

FIG. 2 is a cut-away plan view of the edges of the real and reflected antenna elements and the materials sandwiched therebetween;

FIG. 3 is a perspective view of a vertically polarized log periodic antenna element positioned in a cavity dug in the surface of the ground;

FIG. 4 is a cut-away plan view of the edges of the real and reflected vertically polarized antenna elements and the materials sandwiched therebetween;

FIGS. 5, 6, 7, 8, 9, and 10 show other configurations of log periodic antenna elements that can be adapted for use in a manner similar to the structures of FIGS. 1 and 3 in lieu of the configurations shown in FIGS. 1 and 3;

FIGS. 11 and 12 show specifically how the structures of FIGS. and 6 can be adapted for use in a manner similar to the structure shown in FIG. 3; and

FIG. 13 shows a means for supplying signal to the antenna elements without introducing distortion therein.

Referring now to FIG. 1, there is shown a log periodic antenna element 30 which is positioned over the wedgeshaped cavity 31 and, further, is located in the ground surface plane. The antenna element consists of two radial sections 32 and 33 located on either side of the conductive boom 34 and lying in the same plane. The outer edges of the antenna element 30 are defined by the angle a with the boom 34 bisecting said angle a. As indicated hereinbefore, the distance from the vertex 35 to a point on any given tooth bears a ratio 7' to the radial distance from the vertex 35 to a corresponding point in the next adjacent tooth farthest removed from the vertex 35. A tooth in the antenna element of FIG. 1 is defined as the trapezoidal section formed on either side of the boom 34. Thus, monopoles 36 and 37, together with the end connecting element 38, form a tooth. From the foregoing statements the following relation can be stated:

The cavity 31, as can be seen from FIG. 1, is wedgeshaped and has a sloping bottom 39, two vertical sides, such as vertical side 40 and a vertical end wall 41. The cavity may be formed by conventional construction methods, such as with the use of bucket and crane and bulldozer. It should be kept in mind that some of these log periodic antenna elements run rather large so that the length of the monopole element, such as monopole 36, might be 30 feet or more. Thus, it can be seen that the use of conventional construction equipment is quite suitable in forming the, cavity. The sloping bottom 39 of the cavity is covered with a conductive sheet such as, for example, a copper screening 42, (only a portion of which is shown to avoid congestion in the drawing). Screening is used because it is much easier to cover an area of the size involved with a copper mesh or copper screen rather than to use a solid copper sheet which would not give any appreciably better electrical characteristics. The space between the copper screening 42 and the antenna element 41 can be filled with a dielectric material, such as polyethylene or distilled water. The function of the dielectric material is to increase the electrical length between the antenna and the cavity bottom 39 without increasing the actual physical length. The advantages of 1 such an arrangement will be discussed later herein.

The structure just described above may be more clearly understood from an examination of FIG. 2 which shows a side view of the structure of FIG. 1. In FIG. 2 the antenna element 30' can be seen to be separated from the screen mesh 42 by the dielectric material 43 which fills the wedge-shaped cavity. The line 44 represents the ground surface level and the dotted portion 45 represents the ground around the cavity.

The antenna element 30' and the insulated material 43 are reflected in the screen mesh 42 so as to produce an image 48 of the antenna element 30' and an image 49 of the insulated material 43. Disregarding the boundary condition of ground the radiated pattern in the H plane would be a split beam pattern, the two beams being represented by the reference characters 50 and 51. However, the lower beam 51 is almost completely lost due to the earth and could easily be entirely lost by selecting the parameters of the circuit so that said beam 51 would fall entirely below the ground surface.

It is to be noted that because the copper mesh 42 coupled with the ground beneath functions as a reflector, both the antenna element 30 and the dielectric material 43 are reflected therein. The electrical length then existing between the real antenna element 30' and the reflected antenna element 48 is equal to L Hr while x is the wavelength of the signal, C is the velocity of light, 1 is the frequency of the signal, and K is the relative dielectric constant of the dielectric material 43. If polyethylene, which has a dielectric constant of about 2.2, is employed as the dielectric then the electrical length would be decreased by a factor of .68. This simply means that the depth of the cavity when filled with polyethylene would only have to be about /3 the depth if filled with air to obtain the same electrical separation between the real and virtual antenna elements. It will be apparent that if the desired electrical distance between the real and the virtual antenna elements is a half wavelength and if the lowest frequency is 7 mc., then the quarter wavelength would be approximately 35 feet. With the use of polyethylene it would only be necessary to excavate to a maximum depth of 68% of 35 feet, or about 23.8 feet.

Referring now to FIG. 3, there is shown an alternative form of the invention employing vertically polarized antenna elements rather than horizontally polarized antenna elements shown in the structure of FIG. 1. More specifically, in FIG. 3 the radiating elements are the vertically positioned monopoles, such as monopoles 60, 61, 62, and 63, which are positioned along a conductive boom 59 which, in turn, is located just above the bottom surface 67 of the cavity 68 which, like the cavity shown in FIG. 1, is wedge-shaped. The horizontally positioned elements, such as elements 64, 65, and 66 do not function as radiating elements but rather function as loading impedances in a manner that will be described in more detail later herein. The entire real structure of FIG. 3, including the vertically positioned monopoles, the boom 59 and the horizontally positioned elements 64, 65, and

66, is reflected in the bottom 67 of the cavity 68. Such reflecting surface is created in the same manner as it is in FIG. 1. Specifically, the bottom surface 67 is covered with a conductive mesh, such as a copper screening, with the antenna element being positioned just above the copper mesh, but not making electrical contact therewith.

Each of the horizontally positioned elements, such as elements 64, 65, and 66, then has a corresponding image element in the reflecting surface 67, each horizontal element and its reflected image functioning as an open-circuited transmission line stub having an electrical length in accordance with the frequency of the signal being transmitted. The specific effect of these transmission line stubs is not precisely known. It must suffice to state that they perform a loading function which results in the vertical monopoles, such as 60, 61, 62, and 63, together with the reflected images of the vertical monopoles, producing an electromagnetic field pattern radiated off the vertex 69 of the antenna structure, which pattern is substantially independent of frequency in approximately the same degree as other log periodic antenna systems. The general E plane configuration of such radiation pattern is shown in FIG. 4. The lower lobe shown in the pattern of FIG. 5 is, of course, nonexistent owing to the earth being a boundary condition. For a more detailed description of the operation and characteristics of the vertically polarized antenna element, as shown in FIG. 3, and the effect of the loading of the transmission line stubs, reference is made to copending application entitled Vertically Polarized Unidirectional Log Periodic Antenna Over Ground by David G. Berry, Serial Number 31,068.

Similarly, as in the case of FIG. 1, the cavity shown in FIG. 3 can be filled with a dielectric to reduce the physical depth of the cavity. For example, if the cavity is filled with polyethylene the depth of the cavity need only be 68% of what it would be if it were filled with air. As a further example of the effect of filling the cavity with a dielectric, assume that the cavity is waterproofed and then filled with distilled water which has a dielectric constant of 78. In this case the depth of the cavity is decreased by a factor of 8.8. Thus, for an antenna whose minimum frequency is 7 mc., the maximum cavity depth need only be about 8 feet.

In FIG. 4 there is shown a plan view of the structure of FIG. 3, including the real antenna element of FIG. 3, and the image antenna element, which image element is not shown in FIG. 3. The vertically positioned monopole elements 70 of the real antenna element have reflected images 71 on the lower side of the reflecting surface 67 which consists of conductive material, such as a copper screen, laid on the bottom surface of the cavity. It is to be noted that the dielectric material 72 which fills the cavity also has an image below the reflecting surface 67 for the same reasons as discussed in the structures of FIGS. 1 and 2.

Other configurations of log periodic antenna elements may be used in place of the specific ones shown in the structure of FIGS. 1 and 3. More specifically, an antenna element having triangularly shaped teeth formed of rods, such as shown in FIG. 5, may be employed in FIG. 1. Also, any of the structures shown in FIGS. 6, 7, 8, 9, and 10 may be employed as the real antenna element in FIG. 1 in place of the specific one shown in FIG. 1.

Similarly, all of the structures shown in FIGS. 5 through 10, inclusively, may be adapted to be employed in the structure of FIG. 3. To be so adapted the antenna elements are folded along their center lines so that the two radial sections making up each antenna element are at right angles to each other. Thus the structure of FIG. 5, when adapted to the vertically polarized form of the invention shown in FIG. 3, would look like the structure shown in FIG. 11 and have triangular teeth, such as teeth 74, 75, and 76, lying in a plane parallel to the bottom surface 67" of the cavity 68 and at right angles to the vertically polarized teeth, such as teeth 77 and 78. The bottom surface 67" of the cavity is covered with a sheet of metallic material, such as a copper screen.

Similarly, the structure of FIG. 6 may be adapted to the modification of the invention shown in FIG. 3 in the manner illustrated in FIG. 12. In FIG. 12 the vertically polarized teeth, such as teeth 80 and 81, are positioned perpendicularly to the bottom surface 67" of the cavity 68" and are at right angles to the teeth elements, such as elements 83 and 84, which lie in a plane parallel with the cavity bottom surface 67". In both the structures of FIG. 11 and FIG. 12 the teeth which are parallel to the bottom surface 67 of the cavity 68 function as transmission line stubs and perform a loading function as indicated hereinbefore. Further, it is to be noted that the antenna elements shown in FIGS. 11 and 12 are elevated slightly above the ground surface 67 and insulated therefrom so that they do not make electrical contact with the ground. An image of the structures of FIGS. 11 and 12 is produced in the underside of the conductive bottom surface 67"" in exactly the same manner as discussed in FIGS. 1 and 2.

In a manner similar to that shown in FIGS. 11 and 12 the structures of FIGS. 7, 8, 9, and 10 may be adapted to the modification of the invention shown in FIG. 3.

It should perhaps be noted that in the structure of FIGS. 8, 9, and 10 the transverse dipole elements are of a zigzag configuration. Such a zigzag configuration functions to increase the effective electrical length of the dipole elements in the manner described in United States patent application, Serial No. 63,372, filed October 18, 1960, by Vito P. Minerva and entitled Antenna, now Patent No. 3,106,714, which is incorporated as reference in part in this specification. In general, the effect of the zigzag configuration can be employed in the following manner. A dipole element normally presents a capacitive reactance to a signal having a frequency less than the resonant frequency of the dipole element. Thus, if the capacitive reactance of the transverse dipole element can be reduced, the resonant frequency of the transverse dipole element also will be decreased. Since capacitive reactance is increased proportional to capacitance and since capacitances are run in parallel, it follows that the capacitive reactance of a transverse dipole element and thus the frequency of resonance will be decreased if shunt capacitance can be created across the transverse dipole element. The zigzag configuration functions to create this shunt capacitance between the individual portions of the zigzag dipole element, thus resulting in effectively lengthening the electrical length of the antenna transverse dipole element without increasing the actual physical length of the element. Such effective increasing of the electrical length of the antenna elements will function to lessen the Width of the cavity required in the structure of FIG. 1 and will function to lessen the depth of the cavity required in the vertically polarized structure of FIG. 3.

It is to be noted that the various configurations shown in FIGS. 5 through 10 have radial distances denoted by two or more of the following notations:

m na n+1s n+1 The relationship between these various distances is the same as discussed in connection with the structure of FIG. 1, which is as follows:

vertically polarized antenna elements, such as shown in FIGS. 11 and 12, can be supplied with an input signal in the manner similar to that shown in FIG. 13.

A coaxial cable 91 is brought in underground to a point near the apex 97 of the antenna element. The outer conductor 92 of the coaxial cable 91 terminates at ground level and is connected to the conductive sheet 94, while the inner conductor 93 continues on and is connected to the vertex 97 of the antenna element 90. Since the coaxial cable is buried underground it can be brought in from any direction without disturbing the radiated pattern.

It is to be noted that the forms of the invention shown and described herein are but preferred embodiments thereof and that other log periodic antenna configurations may be employed without departing from the spirit or the scope of the invention.

What is claimed is:

1. An antenna system comprising a body of conductive material having at least one substantially planar surface with a wedge-shaped depression of a general triangular configuration form therein, said wedge-shaped depression defined by three wall surfaces and a bottom surface in said body, each of said three wall surfaces being substantially perpendicular to the said planar surface of said body, said bottom surface lying in a plane which is sloped with respect to said planar surface at a predetermined angle, a triangularly shaped log periodic antenna element having a center conductive boom and being positioned within said depression with the vertex of said antenna element lying near the midpoint of the intersecting line of said bottom surface and the planar surface of the body of conductive material, and means for supplying an input signal across said log period antenna element and said body of conductive material.

2. An antenna system in accordance with claim 1 in which said log periodic antenna elements comprise a pair of radial sections, each radial section having a side comprised of said center conductive boom and being generally triangular in shape and having a common vertex, each of said antenna elements lying substantially in the plane of said planar surface and comprising a plurality of teeth extending outwardly from said center conductive boom, the teeth of each radial section being positioned opposite the gap between the teeth of the other radial section, and the outer extremity of said teeth being defined by the angle a/2, with respect to the center conductive boom.

3. An antenna system in accordance with claim 2 in which the teeth of each of said radial sections are trapezoidal in shape and are comprised of a conductive rod bent to form said teeth, the portions of the conductive rods extending outwardly from said center conductive boom being substantially parallel with each other.

4. An antenna array in accordance with claim 2 in 8 whicla the teeth of each of said radial sections are triangular in shape and are comprised of a conductive rod bent to form said teeth.

5. An antenna system in accordance with claim 1 in which said log periodic antenna element comprises a first radial section and a second radial section positioned about said center conductive boom in planes that are space positioned with respect to each other, said center conductive boom being positioned just above and substantially parallel to the said bottom surface, the first of said radial sections being positioned so as to lie in a plane substantially perpendicular to said planar and said bottom surfaces and constituting the radiating portion of the antenna system, the second radial section of said antenna element being positioned substantially parallel with said bottom surface and constituting, together with its reflection in said conductive body, loading means for said antenna system.

6. An antenna system in accordance with claim 5 in which the teeth of each of said radial sections are trapezoidal in shape and are comprised of a conductive rod bent to form said teeth, the portions of the conductive rods extending outwardly from said center conductive boom being substantially parallel with each other.

7. An antenna array in accordance with claim 5 in which the teeth of each of said radial sections are triangular in shape and are comprised of a conductive rod bent to form said teeth.

8. An antenna system in accordance with claim 5 in which each of said teeth is comprised of a single conductive rod extending outwardly from said center conductive boom.

9. An antenna system in accordance with claim 1 in which said antenna element is comprised of a single radial section positioned in a plane substantially perpendicular to the said planar and bottom surfaces of said depression with the center conductive boom positioned just above and substantially parallel to the said bottom surface of said depression and further comprising impedance loading means between each of said teeth of said radial section, each of said teeth of said radial section being comprised of a single conductive rod extending outwardly from said center conductive boom.

References Cited by the Examiner UNITED STATES PATENTS 2,661,466 12/1953 Barret 343719 X 2,984,835 5/1961 DuHamel et al 343908 2,990,547 6/1961 McDougal 343792.5 X 3,059,234 10/1962 DuHamel et al. 343792.5 3,106,714 10/ 1963 Minerva 343-792.5

HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, Examiner. 

1. AN ANTENNA SYSTEM COMPRISING A BODY OF CONDUCTIVE MATERIAL HAVING AT LEAST ONE SUBSTANTIALLY PLANAR SURFACE WITH A WEDGE-SHAPED DEPRESSION OF A GENERAL TRIANGULAR CONFIGURATION FORM THEREIN, SAID WEDGE-SHAPED DEPRESSION DEFINED BY THREE WALL SURFACES AND A BOTTOM SURFACE IN SAID BODY, EACH OF SAID THREE WALL SURFACES BEING SUBSTANTIALLY PERPENDICULAR TO THE SAID PLANAR SURFACE OF SAID BODY, SAID BOTTOM SURFACE LYING IN A PLANE WHICH IS SLOPED WITH RESPECT TO SAID PLANAR SURFACE AT A PREDETERMINED ANGLE, A TRIANGULARLY SHAPED LOG PERIODIC ANTENNA ELEMENT HAVING A CENTER CONDUCTIVE BOOM AND BEING POSITIONED 